In addition to a lifting-device brake system having exactly one brake assembly that can be activated by a spring force, the present invention thus also relates to a lifting-device brake system having more than one brake assembly that can be activated by a spring force, for example, exactly two, exactly three, or more brake assemblies that can be activated by a spring force.
Each brake assembly that can be activated by a spring force can in turn comprise exactly one or a plurality of brakes, for example, exactly two, exactly three, exactly four, or more brakes. The brakes can, for example, be disk brakes.
Lifting devices of the type here discussed are used to lower and raise heavy loads, particularly of firmly anchored or floating platforms, to or from the seabed.
Such lifting devices always comprise brake systems so that the lowering speed can be controlled and, if necessary, stopping can be performed as quickly as possible (“emergency stop”).
Such brake systems are designed to be activated by a spring force in order to provide the greatest possible operating reliability. This means that the pressure via which stationary actuating elements are pressed against rotating parts of the brake assembly, in the case of disk brake assemblies, the brake linings against the brake disk, is provided mechanically via a spring force. In order to reduce the braking force, piston/cylinder assemblies that can be hydraulically or pneumatically actuated are then provided which act against the spring force when pressure is applied and via which the pressure, under which the stationary parts lie against the rotating part of the brake assembly, thus can be varied. Such a brake assembly design has the advantage that, when irregularities that lead to pressure losses occur in the hydraulic or pneumatic assembly, the brake assembly is not released, but is rather blocked. When the “emergency stop” is triggered, it is merely necessary to relieve the pressure present at the piston/cylinder assemblies, which can be accomplished, for example, by opening a return valve or a discharge valve.
Such a lifting-device brake system can, however, prove to be problematic if the lifting device is used on floating platforms in conjunction with systems for wave compensation (“active heave systems”). In this case, the motion of the platform relative to the seabed caused by the wave motion is sensed and the lifting device is controlled so that the load is at least approximately stationary in relation to the seabed. If, in this case, the brake assembly were also activated when an “emergency stop” that shuts down the entire installation is activated, the load carried by the lifting device would be set into the motion of the platform caused by the wave motion, and, because of inertia, this would lead to high forces acting on the load, the lifting device, and the platform. This circumstance is especially problematic if the lifting device is used to raise and lower a drill string that is firmly connected to the bottom (“locked to bottom”). In this case, activation of the brake assembly would result in either the string being torn off, in the case of an upward motion of the platform, or the platform not being able to follow the wave motion, whereby in the worst case the platform would be pulled under water.